Fix #29: Issues linking to check

https://github.com/alobbs/libhpack/issues/29

It has been detected that when pkg-config is not installed on the
machine you will get a series of linking errors:

```
Linking C executable test_libchula
cd /home/akrog/Web/Teamwork/libhpack/build/libchula/test && /usr/bin/cmake -E cmake_link_script CMakeFiles/test_libchula.dir/link.txt --verbose=1
/usr/bin/cc      CMakeFiles/test_libchula.dir/avl_test.c.o CMakeFiles/test_libchula.dir/main.c.o CMakeFiles/test_libchula.dir/list_test.c.o CMakeFiles/test_libchula.dir/util_test.c.o CMakeFiles/test_libchula.dir/buffer_test.c.o CMakeFiles/test_libchula.dir/missing_sysfuncs_
test.c.o CMakeFiles/test_libchula.dir/md5_test.c.o  -o test_libchula -rdynamic ../libchula.a
```

That is because cmake is piggybacking on pkg-config for the resolution
of *pkg_check_modules(CHECK check)* and since there is no pkg-config it
cannot do the search.

I believe we should set pkg-config and check as REQUIRED in cmake so
that the compilation stops as soon as this is detected.

This solution will work fine (it will compile correctly) if there is no
pkg-config but we have selected BUILD_TESTS=OFF in the Makefile.